Compensated amplitude discriminatory circuits



B. E. BENTON Sept. 23, 1952 .COMPNSATED AMPLITUD DISCRIMINATORY CIRCUITSFiled Feb. 26, 1949 INVENTOR Edward eizo BY 0./

AT'TORNEY Patented Sept. 23, 1952 UNITED 2361113821 nooMPEN'sA'rEnlnmm'rUnE DISCRIM- INAfroRY cmoui'rs Betiieifndwaira Demon,Merfeiianfsvilign. J2, asjsignor to Rfad-o Corporation o'f Aieica aicoi'' poration' of Belaware l y Appueaartmm ze, 19411; stiamo. 7155112 1The present inventionl r`e 1a`tes` to` threshold or gate circuits andmoreparticularlyl to those circuits known asclipping circuits wherein a'jx'd percentage of an applied input wavel isiv to' be translated by thethresholdcircuit.

In one of its'morespecic fornris,' v the" present invention concerns'itself1 with an improvedY synchronizing signal separator for use-withtelevision receiving circuits 'adaptedtohandlestandL ard typestelevision signals; the"inventio n" pro'- viding substantially "constantpercentage' clip'- ping action regardless o'f variationslofapplie'dlinput signals.

In the electrical artespecially"` thatf` phase dealing withcommunication equipmentgfit' is often desirable to provide athreshold"or gatecircuit which will pass ar co-nstalr'itf percentagefof appliedinput signals". Pa`rticularly4 do such loir'- cuits find extended use intliefieldjofl television wherein the compositetelevision-signal:is'lo'ften broken down into itsv video signalcomponent and its synchronizing; component. As is Well known to thoseskilledintliefart, injthe cernposite video signal, the`synchronizingcomponentr amplitude extends substantiallybeyond-the:mostl remote excursions of' the vido signal; I Accord' ing tocommon television radic'- transmission standards, the peaks ofsynchronizingu signals'r'epf'` resent substantially 100% of'A thetransmitter amplitude modulation whereas" the highest am'- plitudeexicursions Qf the'videoI signals4 (and blanking) are limited toapproximately-'75% of transmitter amplitude modulation. In order toproperly reproduce `the-"television image at -a re-v ceiving location,itis-necessary to separatethe synchronizing component from-thevideosignal component.

It therefore can be seen thatfa-circuitfor properly separating thesynchronizing components from the composite signal must:v be respon-vsive to pass not much more'i'than- 25% of the demodulated video-signalIn orderto satisfac-v torily accomplish thlsf lxedA percentagesynchronizing signal separation'.- under`v conditions where thereceivedrsignal varies considerably in' amplitude, it isnecessaryduring" circuit'op'eration to alter the threshold on theclipping". cir-V cuit so as to 'compensatefor the 'changesfin signalamplitude.

It is further common practice-in television re-v ceivers to limitreceived signalamplitudesin or' der to minimize the efiects'ofspurious*noiseon` cuits taking manyfwellknown" forms; some'of" teachingsof the following descr'ipt Itis therefore@ purpose or the prsent in"11:- tion tdprovide an' amputdefdisrnunatr gte circuitv which operatesto' communicate' onlyv a xed percentage ofi applied signal.

l It" isf another purpose of^` the present ifrv'er'- tio'r1 to provide'asimple" and'l no'vel clipper c cuitgfof removing' ai substanuauy nxdper tage of' the; higher excursiciifi-` of; aril applied sign over'averyWide range of' s'ignalo'u'ter amplitude levels'.

It isffurth'er a: purpose of ti1e^piiesentj-i toprovide-ai simpleandeco'rio'niicalsynch ing signal clipping circuitii'orus'in telev iireceivingl systemswherein.fV the synchro "zing v111i: formation'cuppedty" the circuit' remains res'entative-of a fixed pece'ri'tagA of:comp video g signal regartiles's'y of amplitude variations in theapplied comps'itesign'aL t is sun-fuk ner aflp'iirp'qse dfjtiive'lfesefiit@in'-v verition to provideg ai novel* synchr" nifnvgi signalclipping" circuit for televisio`n`- rce vingcircuits which utilizes aminimunof-fciic and yet?lrrySl the functie cathode resistor 26 which isproperly lay-passed by capacitor 28. A variable bleeder resistor 36 isthen connected from a source of positive potential 32 to the upperterminals of the cathode resistor 26 to provide a conventional form ofbrightness control for the reproduced image. 'Ihe video ampliiier i6 hasits cathode 34 connected with ground through a cathode resistance 35across which will, of course, appear a sync positive version of thedemodulated video signal i8.

According to the form of the present invention shown in the drawing, adiode 38 is connected from the cathode 34 of the video amplifier It tothe cathode 48 of a second electron discharge tube 42, the anode 44 of,the diode 38 being connected with the cathode of the video ampliiienThe cathode of discharge tube 42 is then connected through a controlresistor 46 to a source of negative potential 48. An integratingcircuit, such as shown lby capacitor 58 and resistor 52, is connected inshunt with .the control resistor 46 through series resistor Y54. Theintegrating circuit side 36 of the series resistor 54 is shown as beingconnected with the control electrode 58 of the discharge tube 42. In aconventional manner, the anode 66 of the discharge tube 42 is suppliedwith a positive .potential through an output load impedance 62. Theseparated synchronizing pulses are then coupled with capacitor E4 to thecontrol grid 66 of a sync limiter 68. The input resistance of the synclimiter 68 is connected from the control electrode 66 to the cathode 16.The limited sync appearing across the output resistance I2 of thelimiter 68 is applied to some conventional sync separator circuit, suchas shown at 'I4 through coupling capacitor 76. In the sync separatorcircuit 14, the vertical synchronizing signals are separated from thehorizontal synchronizing signals and respectively applied to thevertical and horizontal deflection circuits 78 and 80 with respectiveoutput terminals Y-Y and X-X provided for connection to the appropriatebeam deection coils 82.

Detailed operationlof the above circuit in accordance with the presentinvention is substantially as follows: The sync positivesignals'developed across the cathode resistor 35 of the video amplifierI6 are applied to the anode 44 of the diode 38. Due to screen and .anodecurrent of the disch-arge tube 42 through the control -resistor 46, thepotential applied to the cathode of the diode is adjusted to besubstantially more positive than the potential statically appearing atthe diode anode 44.

Thus, only the more positive excursions of the composite video signal,which may correspond to its synchronizing component and aportion of theblanking pedestal, will cause thediode 38 to conduct. Consequently, the`waveform passed by the diode 38 Will be substantially that shown at 84which in turn is applied to the cathode 40 of the discharge tube 42. The`discharge tube 42 is thus operated as a cathode driven amplifier and anamplified version 86 of the same positive sync polarity Will appearacross the load resistor 62 in its output circuit.

The signal 86 is then capacitively coupled to the grid 66 of the limiterstage 68. Inasmuch as there is no control grid bias on thevvacuurn tube68, the positive peaks of the amplified separated sync 86 will Ithencause grid current conduction by the control grid 66 which will tend ltolimit any spurious noise which might extend beyond the positive extentof the applied clipped sync.

Furthermore, since the grid lcurrent will tend to charge the capacitor64 such as to develop a variable negative bias on the control grid 66, asecond well known clipping action may be obtained to provide across theoutput resistor 'l2 a negative sync signal 88 more completely free ofblanking pedestal or lower amplitude composite signal component.

If the received signal intensity drops to produce a decrease in theamplitude of the dernodulated signal I8, the novel arrangement of theapplii cants invention will act to maintain the clipped signal 84 at asubstantially const-ant percentage of the overall composite signal I8.This action can be understood since a portion of vthe clipped signal 84has been applied to the integrating circuit comprising condenser 56 andresistor 52, so that voltage thereacross will vary in accordance withthe amplitude of the clipped signal 84. Under static conditions, Ithegrid 58 of the vacuum tube 42 is maintained at a negative potential withrespect tothe cathode 40 by an amount equal to the D. C. voltageappearing at lthe junction 36 of resistor 52 and 54 which eiiectivelydivides the voltage appearing across the control resistor 46. Undernormal reception, the voltage at the grid 58 becomesslightly morepositive because of theintegration of the clipped signal 84 across theintegrating circuit 56 and 52. Should then the signal I8 decrease,v theenergy represented by the clipped signal 84 will be reduced and thevoltage on the grid 58 of the vacuum tube 42 will become more negative.This will decrease the current through the control resistor 46 which isconnected in series With the diode 38 thereby making the diode-cathode45 more negative. Consequently, the diode 38 will clip more deeply intothe composite signal developed across the resistor 35 and tend tomaintain .the percentage of clipping constant.

From the foregoing it can be seen in the above arrangement that theapplicant has provided a simple, novel and effective combination circuitfor providing approximately constant percentage sync clipping oftelevision signals, amplification of the clipped signals as well as asecond limiting and clipping function to provide a separatedsynchronizing signal virtually free from signal fading affects and noiseof greater amplitude than the clipped signal.

` What is claimed is:

An amplitude discriminatory circuit for communicating a substantiallyxed percentage of the more positive extent of an applied signal andcomprising: an input terminal for the applied signal; a resistanceconnected from said terminal to a rst point of iixed potential; anelectron discharge tube having at least an anode, a cathode, and aninput electrode; an impedance in the anode-cathode circuit of saiddischarge tube, said impedance being also connected with a second pointof iixed potential, a unilateral conduction device having an anode and acathode, a connection from said unilateral conduction device anode tosaid input terminal; a connection from said unilateral conduction devicecathode to a point on said discharge tube anode-cathode circuitimpedance, saidconnection being such to include a portion of saidanode-cathode impedance as a portion of a load circuit for saidunilateral conduction device; an integrating circuit having one terminalthereof connected to said second point of xed potential, an impedanceconnected from another terminal of said integrating network to saidconduction device cathode, means including a connection from saiddischarge device input electrode to said last named impedance forcontrolling the average current through said electron discharge tube inaccordance with the value of the integrated Voltages across saidintegrating circuit thereby to alter the current through said dischargetube anode-cathode impedance in accordance with the value of developedintegrating circuit voltage such to maintain the amplitude of the signalappearing at said unilateral conduction device cathode at asubstantially fixed percentage of the signal applied to said inputterminal regardless of amplitude Variations therein. v

B. EDWARD BENTON.

REFERENCES CITED The following references are of record in thev le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,181,572 Bowman-Manifold et al.Nov. 2-8, 1939 2,279,007 Mortley Apr. 7, 1942 10 2,298,084 Fyler Oct. 6,1942 FOREIGN PATENTS Number Country Date 115,307 Australia June 25, 1942OTHER REFERENCES Wireless World, Feb. 23. 1939, pages-174-177.

